Economic production of argon via air separation plants is linked to the production of equivalent quantities of nitrogen or oxygen or both. In recent years, however, the demand for argon has been growing at a more rapid rate than the corresponding growth rate of either nitrogen or oxygen. Alternative sources for argon production have thus become attractive. One such alternative source is the purge gas from ammonia synthesis plants.
In an ammonia synthesis plant, it becomes necessary to purge a fraction of the gas stream in order to maintain the inert concentration below a specified level. Higher inert levels reduce the partial pressure of the reactants and cause an unfavorable shift of the ammonia synthesis reaction equilibrium. Methane and argon constitute the inert gases of concern. A typical composition of the ammonia purge gas available at approximately 1900 psig pressure is as follows: 60.5% H.sub.2, 20% N.sub.2, 4.5% Ar, 13% CH.sub.4 and 2% NH.sub.3. Depending on the ammonia plant design, the purge gas may be available at much higher pressures or at slightly different compositions.
Present technology for argon recovery from ammonia plant purge gas employs a cryogenic process that consists of a pre-treatment section for ammonia removal and three cryogenic distillation columns. The first two columns are for stripping hydrogen and nitrogen in the feed gas and the final column is for separating argon and methane to obtain pure liquid argon product and also pure methane for use as fuel.
The primary object of the invention was to develop an improved process for recovering argon from ammonia synthesis plant purge gas. A further object of the present invention was to develop a process employing an advantageous combination of non-cryogenic and cryogenic steps for argon recovery from an ammonia synthesis plant purge gas. Yet a further object of the present invention was to utilize a PSA system to accomplish removal of methane from the purge gas exiting an ammonia synthesis plant.
In the following description of the invention, the term "pressure swing adsorption" or its acronym "PSA" is used in reference to a type of process and apparatus that is now well known and widely used with respect to separating the components of a gaseous mixture. A PSA system basically comprises passing a feed gas mixture through one or more adsorption beds containing a sieve material which has greater selectivity for more strongly adsorbed components than more weakly adsorbed components of the gas mixture. In the normal operation of a typical 2-bed PSA system, the connecting conduits, valves, timers, and the like are coordinated and arranged so that when adsorption is occurring in a first bed, regeneration is occurring in a second bed. In the usual cycle, sequential steps with respect to each adsorption bed include bed pressurization, product release and bed regeneration. Basic PSA systems are described in U.S. Pat. Nos. 2,944,627, 3,801,513, and 3,960,522.
Various modifications and improvements to the basic PSA process and apparatus have been described in the literature, for example, in U.S. Pat. No. 4,415,340, issued on Nov. 15, 1983 and U.S. Pat. No. 4,340,398 issued on July 20, 1982. The present invention is not limited to the use of any particular PSA process or apparatus design. A design that results in high argon yield, however, is detailed below as an example.
A new and improved process has been developed for recovering argon from the purge gas flowing from an ammonia synthesis plant. This process employs a non-cryogenic means comprised of a pressure swing adsorption (PSA) unit for accomplishing a critical separation between argon and methane as well as removing most of the nitrogen.
The present invention has several important advantages over the three stage prior art cryogenic recovery of argon. A considerable reduction in capital cost and operating expense is achieved through the use of a gas phase methane separation. In fact, the high pressure of the purge gas exiting from the ammonia plant can be used to provide most or all of the energy requirements in the non-cryogenic separation. Furthermore, it is possible, as a further energy saving measure, to pass the purge gas stream through a turbine in order to provide cooling needed for the later cryogenic separation. Further advantages of the present process stem from the use of PSA units for ammonia separation and for methane separation ahead of a membrane for hydrogen separation. The ammonia purge stream is at a cold temperature (about -10.degree. F.), at which pressure swing adsorption is more effective. Because of the heats of adsorption, the product gas from the PSA units will be warmer. Membranes operate more effectively at higher temperatures (about 70.degree. F.). The temperatures noted here are for an ammonia plant operating at about 2000 psia and may be slightly different in the event the plant is designed to operate at other pressures.
Another advantage of the present invention is that it offers the option of separating ammonia simultaneously with methane and nitrogen in a single PSA system. This option is advantageous when ammonia in the feed is at such low concentrations that the recovery or recycle of ammonia in the purge gas is not critical. Expensive ammonia separation equipment are then eliminated, making the process even more cost effective. Finally, the compact units employed in the present process are more portable and, as a result, the purge gas available at numerous ammonia plant sites over a wide geographical range can be more expeditiously tapped for argon in order to meet the growing demand for this industrial gas.